This invention relates to a circuit substrate and a thermal printing head using the circuit substrate, and more particularly to a circuit substrate provided with fine electrodes suitable for soldering, that is, the so called "microsoldering" and a thermal printing head using the said circuit substrate.
The conventional electrodes for solder bonding, provided on a circuit substrate, are formed by laying an adhesive layer 2 (mainly of chromium) and a barrier metal layer 3 (for example, of Cu, Cu+Cr, Ni, Pd, Al or Rh) successively on a circuit layer 1, as shown in FIG. 5, and, if desired, by further laying an oxidation protective film of Au, etc. thereon, where numeral 4 is a solder. The thickness of electrodes and the adhesion of the solder depend upon the properties of a barrier metal.
For example, Cu is dissolved from the Cu barrier into the molten solder in an amount corresponding to about 1 to a few .mu.m of the Cu barrier layer thickness by only one run of soldering, and thus it is at least necessary that the Cu barrier layer has a larger thickness than the corresponding dissolved Cu layer thickness, and, when solder bonding operation is once more carried out, melting of the solder is to be carried out in a total of two runs, for example, when the initial solder bonding is removed and when second bonding by solder is carried out, Cu is to be dissolved from the Cu barrier layer into the molten solder in a double amount, and thus, the actually necessary thickness of the barrier metal layer is 3 to 4 times the corresponding dissolved layer thickness by one run of soldering, and in the case of Cu barrier metal, the necessary layer thickness will be at least 3-4 .mu.m.
When such a thick barrier metal layer is formed on a substrate, breakage of the substrate or cracking of the barrier metal layer itself is liable to occur owing to a thermal stress due to a difference in the coefficient of thermal expansion. Furthermore, when an electrode-protective layer is formed on the electrodes, some defects are liable to occur owing to a level difference between the substrate and the electrodes. In the case of other barrier metals than Cu, the necessary thickness of such barrier metal layer is smaller than that of the Cu barrier-layer, but a bonding failure often occurs owing to the poor adhesion to the solder. Furthermore, Pd and Rh are very expensive as barrier metals.
To solve these problems, Sharp Giho (Technical Bulletin), No. 33, pages 55-60 (1985) discloses use of a Cu--Ni alloy, as shown in FIG. 6(a), where the Cu13 Ni alloy layer is formed to a thickness of 1 .mu.m as a circuit and barrier metal layer 6, whereby the layer can withstand two reruns of bonding (that is, three runs of soldering). In FIG. 6(a), numeral 1 is an aluminum thin film, 2 a lower circuit layer, 5 a substrate, 6 an upper circuit layer, 7 a polyamide-imide resin layer, 8 a silicone protective layer and 9 is a gold layer. The Cu--Ni alloy barrier layer having a layer thickness of 0.3 to 0.5 .mu.m cannot withstand 3-4 runs of soldering in contrast to the Rh or Pd barrier layer.
Furthermore, in this case, a resin is used as a protective layer 8 for the barrier Cu--Ni alloy layer, but no mention is made of the adhesion of the barrier layer to a protective layer of inorganic materials such as SiO.sub.2, Ta.sub.2 O.sub.5, etc.
The present inventors have studied Cu--Ni alloy barrier layers having various compositions and investigated the problems of the prior art, and have found that a solder-diffusion prevention can be obtained by forming an alloy barrier layer having a composition of 50 mol. % Ni--50 mol. % Cu on the substrate. However, it has been found that there still are the following problems. That is, the Cu--Ni alloy barrier layer thus formed has a very poor adhesion to the protective layer of SiO.sub.2 formed on the barrier layer by RF sputtering, and the protective layer is easily peeled off from the barrier layer by heating or cooling or the barrier Cu--Ni alloy barrier layer has a poor anticorrosivity to a resist-removing agent to be used during the photoetching, and consequently is easily corroded in the circuit pattern-forming process.
In other words, the prior art disclosed in the said Sharp Giho fails to take into account any application of the barrier metal layer to a fine circuit, and it is impossible to form electrodes having a width of a few ten .mu.m, as required for IC terminals to be soldered, etc., or to form a protective layer of inorganic material such as SiO.sub.2, etc. on the barrier metal layer. Furthermore, the disclosed prior art is directed only to circuits requiring a film thickness of about 1 .mu.m, and thus cannot be applied to IC terminals to be soldered, a thermal printing head, etc., particularly to CCB bonding, owing to the insufficient properties as required for a solder-diffusion protective layer.